1) Field of the Invention
The present invention relates to technology for maintaining the temperature distribution within an optical device uniform.
2) Description of the Related Art
In recent years, the information transmission volume has been increased by the popularization of the Internet, etc. and construction of a high-speed and large-volume network is required irrespective of at home or abroad. In particular, for Japan in which the Internet diffusion rate is flagging at a level of 37%, which ranks as low as the 14th among major countries in the world, early improvement of network infrastructure is a key issue.
Therefore, the Government has been promoting the IT revolution named “e-Japan Strategies” since 2001 to stay well ahead in the race of international competition in the 21st century. In this “e-Japan Strategies,” Japan aims at becoming the world's most advanced IT nation within 5 years, aims to provide high-speed constant access network (several Mb/s) to at least 30 million household and ultra high-speed constant access (30 Mb/s to 100 Mb/s) to 10 million households at extremely low rates, and aims to build an “World Highest Level Advanced Information and Telecommunications Network Society” where everyone can enjoy the IT benefits.
As a means for achieving this “World highest level advanced information and telecommunications network society,” it is assumed that “PhotoniXnetwork Technique” is the most effective and indispensable, and even in the priority plan of “e-Japan Strategies,” after three years, development and promotion of “PhotoniXnetwork technique” are requested, such as materialization of 1000 waves/core wavelength-division multiplexing technique, practical application of 10 Tbps optical router technique, practical application of technique to control and manage the optical network without converting to electrical signals, etc.
And the wavelength division multiplexing (WDM) system is now under development and practical application as a telecommunications system which can remarkably increase the transmission capacity. In each node which composes the WDM system, functions to branch and insert optional wavelength are essential, and as an optical device which can materialize these functions, an acousto-optic tunable filter (AOTF) utilizing acousto-optic effects attracts keen attention of the people concerned and are studied and developed in many institutes. This device provides excellent advantages such as simultaneous selection of multiple wavelengths, wide wavelength selection band exceeding 100 nm, etc. In addition, as an optical device which adds and drops a large number of optical signals, optical add/drop module arrayed waveguide grating is an important key device, too, and provides advantage that a device which matches signal wavelength grid, 0.8 nanometer (nm) intervals to a device which matches to grids ¼ the intervals can be manufactured.
The AOTF construction and operating principle using the LiNbO3 substrate have been already well-known (for example, see non-patent literatures 1 and 2). FIG. 13 is a top view one of an example of the configuration of a conventional acousto-optic tunable filter (AOTF). The acousto-optic tunable filter is sometimes referred to simply as optical filter. A substrate 204 that composes an acousto-optic tunable filter 202 forms optical waveguide 208 by Ti diffusion with X-cut LiNbO3. An acoustic wave is formed by patterning an inter-digital transducer (IDT) 206 that generates acoustic waves. A polarization beam splitter (PBS) 212 is formed by Ti-diffusion etc. The incoming light is polarized and separated by the polarization beam splitter (PBS) 212 into two beams, each beam passes through a surface acoustic wave (SAW) guide 210, the two beams are polarized and synthesized by the polarization beam splitter 212, and a transmitted light is output to a transmitting light port. Only the wavelength corresponding to the frequency of SAW generated by the inter-digital transducer (IDT) 206 is polarized and converted when it passes through the SAW guide 210 and output to a branched light port. Reference numeral 214 denotes an absorber which absorbs the surface acoustic wave (for example, see patent literature 1).
When SAW is excited by acousto-optic effects which the LiNbO3 crystal has, the crystal axis is tilted. With the tilting of the crystal axis, the SAW shifts between positive and negative in a cycle. This one cycle functions as a just ½ wavelength plate for a specific optical wavelength. That is, for an optical signal with a specific wavelength, the microscopic ½ wavelength plate looks like alternately tilting at microscopic angles with respect to the crystal axis. This is exactly the principle of the FSF (folded SoIc filter) (for example, see non-patent literature 3). The light signal moves vibrating on a Poincare sphere by FSF and becomes polarized light orthogonal to the incoming polarized light exactly at the SAW guide output point.
On the other hand, because for optical signals other than the specific wavelength, the condition looks like filters which slightly shift from the ½ wavelength plate aligned in a line, optical signals are averaged and no polarization conversion is generated. Because selective filter characteristics are obtained by such principle, excellent filter characteristics with narrow pass band can be obtained because even a slight wavelength change is filtered as the device length increases. Since this is a device with long device length as described above, various problems occur in the module construction, too.
For various techniques to improve the characteristics of a simple device are known. These techniques include an acousto-optic filter comprising a light waveguide for propagating single relative rectilinear polarized light, a surface acoustic wave (SAW) generating means mounted on the optical waveguide for generating an SAW, and an interaction region which distributes a propagation loss of the SAW spatially and converts a specific wavelength component of the single relative rectilinear polarized light propagated in the light waveguide into rectilinear polarized light which crosses this at right angles (for example, see patent literature 2).
In addition, there is well-known is a light wavelength characteristics adjusting method for adjusting filter wavelength characteristics by changing the shape and location of a strain-providing section after manufacturing an element with the strain providing section for correcting local double refraction index of an optical waveguide (for example, see patent literature 3).
In addition, there also well-known is a wavelength filter with an absorber for absorbing an SAW by each reflective electrode to the outside of the optical waveguide by forming the optical waveguide and excitation electrodes for exciting the SAW on an acousto-optic crystal substrate and disposing reflective electrodes on propagation passage of the SAW (for example, see patent literature 4).
For a soaking structure in a module using a waveguide device, a waveguide type optical module is well known, in which a heating/cooling element for controlling the waveguide type optical element temperature via a soaking plate and heat buffer layer is installed on the waveguide type optical element with temperature dependency and at least part of the soaking plate is brought into contact with the waveguide type element (for example, see patent literature 5).
Furthermore, an arrayed waveguide grating which uses an optical add/drop module comprises, a waveguide chip (including, for example, optical substrate such as silicon, quartz, sapphire, etc.) with an arrayed waveguide (channel waveguide) with optical add/drop functions formed on the surface, a slab waveguide, and a soaking plate which bonds to the rear surface of waveguide chip and soaks waveguide chip, wherein the upper plate for an optical fiber connection is installed to the surface with an arrayed waveguide of a waveguide chip formed (for example, see patent literature 6).
FIG. 14 is a top view of another example of the configuration of the conventional acousto-optic tunable filter. A signal wiring is installed on an inter-digital transducer, when a plurality of optical waveguides are arranged on one substrate, to achieve multichanneling. As shown in FIG. 14, when multichanneling is achieved, signal wiring installation by fine line patterns is adopted. That is, the acousto-optic tunable optical filter 202 composes a plurality of channels (for example, channels 1, 2) using the configuration same as that shown in FIG. 13 on LiNbO3 substrate 204.
Next, FIG. 15 is to explain heat resistance of module construction according to a conventional technique. As shown in FIG. 15, the acoust-optic tunable filter 202 of LiNbO3 waveguide type is generally modularized in the following manner. That is, a heater 224, which is a temperature control section, is fixed to a soaking plate 222, for example, copper plate, etc. intervened on the rear surface of the substrate 204 made of X-cut LiNbO3. The entire structure is housed in a package (PKG) 226. Each optical fiber, etc. for optical signals connected to the substrate 204 are pulled out through insertion holes formed in the package 226. Reference numeral 230 is a lid which covers an opening at the top surface of package 226.
FIG. 16 is a circuit diagram of a heat equivalent circuit based on FIG. 15. As shown in FIG. 16, the heat conductivity in the module construction of the acousto-optic tunable filter 202 can lead through the heat equivalent circuit shown in FIG. 16 with each element of the package construction taken into account as heat resistance. That is, the heat conductivity is assumed to be obtained by connecting in parallel between current source I and external air Ta serially connected heat resistance RLN0i on the nearly center side of LiNbO3 substrate 204 and upper air resistance Rair0 on the nearly center side to serially connected heat resistance RLN1 on the edge side of LiNbO3 substrate 204 and upper air resistance Rair1 on the edge side connected in series.
Following equations hold:
                    Th0        =                              i0            ·            Rair0                    =                                    R1              ·              I              ·                              Rair0                /                                  (                                      R1                    +                    R0                                    )                                                      =                          Δ              ⁢                                                          ⁢              T                                                          (        1        )                                R1        =                  RLN1          +          Rair1                                    (        2        )                                R0        =                  RLN0          +          Rair0                                    (        3        )                                          ∴                                          ⁢          I                =                                            (                              R1                +                R0                            )                        ·            Δ                    ⁢                                          ⁢                                    T              /              R1                        /            Rair0                                              (        4        )                                          Δ          ⁢                                          ⁢          Ts                =                ⁢                  Th0          -          Th1                                    (        5        )                                                          ⁢                  =                    ⁢                                    i0              ·              Rair0                        -                          i1              ·              Rair1                                                                                                                    ⁢                  =                      I            ·                                          (                                                      R1                    ·                    Rair0                                    -                                      R0                    ·                    Rair1                                                  )                            /                              (                                  R0                  +                  R1                                )                                                                        (        6        )                                          Δ          ⁢                                          ⁢          Ts                =                  Δ          ⁢                                          ⁢                      T            ·                          (                              1                -                                                      (                                          R0                      ·                      Rair1                                        )                                    /                                      (                                          R1                      ·                      Rair0                                        )                                                              )                                                          (        7        )            
where, Th0 is the temperature of the substrate 204 at the center, Th1 is the temperature of the substrate 204 at the circumference, and ΔT is temperature difference from the outside.
Patent literature 1: Japanese Patent Application Laid-Open Publication No. 2001-330811.
Patent literature 2: Japanese Patent Application Laid-Open Publication No. H8-146369.
Patent literature 3: Japanese Patent Application Laid-Open Publication No. H11-326855.
Patent literature 4: Japanese Patent Application Laid-Open Publication No. H9-49994.
Patent literature 5: Japanese Patent Application Laid-Open Publication No. 2002-90563.
Patent literature 6: Japanese Patent Application Laid-Open Publication No. 2000-249853.
Nonpatent literature 1: Optorics (1999) No. 5, P155.
Nonpatent literature 2: The Institute of Electronics, Information and Communication Engineers, OPE 96-123, P79.
Nonpatent literature 3: Optical Waves in Crystal, AMNON YARIV, A Wiley-Interscience Publication, P137.
However, the various techniques are described in patent literature 1 through 4 only to improve the characteristics as a simple AOTF device and they do not consider thermal measures when AOTF is modularized, and they cannot solve problems in multichanneling in which a plurality of AOTFs are positioned on one LiNbO3 substrate.
To describe the detail, it is known that in AOTF device, filter characteristics degrade when the temperature distribution is present on the SAW guide surface. The filter characteristics are assumed to be degraded because stress is applied nonuniformly due to temperature distribution, crystal strain by acoustic effects becomes nonuniform, and the relation of microscopic ½ wavelength plate to the specific wavelength excited by SAW is broken down. Consequently, it becomes a big problem how to keep the temperature in the SAW guide uniformly. Furthermore, when multichanneling is achieved, not only the device length but also the width increase, and uniformity of temperature on the device surface causes still more difficult problems.
The configuration to have a soaking plate to AWG is a generally adopted technique as disclosed in patent literature 5, 6, but because in the AOTF device, the device area is large, there is a problem that a temperature gradient is generated due to the temperature gradient of the heater itself, the difference of heat resistances of air on the device surface, etc., and the desired temperature uniformity cannot be obtained with the soaking plate only. The temperature cannot be made uniform by the configuration in which the soaking plate is simply placed on the Peltier element and effects from the outside cannot be eliminated. By the way, when the soaking plate is formed by material with good thermal conductivity, the power consumption of the Peltier element increases.
In this way, since the device area is large with the conventional AOTF device, there is a problem that a temperature gradient is generated due to the temperature gradient of the heater itself, difference of heat resistances of air on the top of the device, etc. and there is a problem that the desired temperature uniformity cannot be obtained by the soaking plate adopted by the conventional technology.
In addition, when the AOTF device is multichanneled, installation of signal wiring to an inter-digital transducer (IDT) becomes an important problem, but in the waveguide device, forming a grand pattern with large area in order to facilitate installation of signal wiring, the light is absorbed by the large-area metal patterns and loss increases. Consequently, installation of signal wiring by fine-line patterns as shown in FIG. 14 becomes necessary. Since by signal wiring installation by this fine-line pattern, line with 50-Ω characteristic impedance cannot be designed, signal patterns become all inductance components, giving rise to problems of degraded RF signals applied and generation of cross-talks between adjacent signal patterns, too. In addition, another problem that the device area increases to secure a signal wiring installation region occurs.